A fuel cell system is an electricity generating system that converts chemical reaction energy of oxygen and hydrogen contained in hydrocarbon fuels, such as methanol, ethanol, or natural gas, into electric energy.
Fuel cell systems include polymer electrolyte membrane fuel cells (PEMFC) and direct oxidation fuel cells among other types. Fuel cell systems employing the PEMFC scheme include a stack which comprises the body of the fuel cell, a reformer for reforming the fuel to generate hydrogen and for supplying the hydrogen to the stack, and an air pump or a fan for supplying oxygen to the stack. The stack generates a predetermined amount of electric energy through a reaction between hydrogen supplied from the reformer and oxygen supplied through the air pump or the fan. In a fuel cell system employing the direct oxidation fuel cell scheme, the fuel is directly supplied to the stack to generate electric energy through an electrochemical reaction of the fuel and oxygen. The direct oxidation fuel cell system does not require a reformer.
In the fuel cell systems described above, the stack includes several to several tens of unit cells stacked next to one another. Each unit cell has a membrane-electrode assembly (MEA) and separators closely located on both surfaces of the MEA.
The separators are provided with a first passage for supplying hydrogen or fuel to the MEA and a second passage for supplying oxygen to the MEA. The first and second passages are formed as channels on the surfaces of the separators. The channels are formed between a plurality of ribs arranged along straight lines and the ends of the channels are alternately connected together creating a continuous path. The ends of the channels are connected through connecting portions.
In a conventional stack, turbulence forms at the corners of the connecting portions which are rectangular and creates a resistance to the flow of hydrogen or fuel and oxygen. Accordingly, in conventional stacks, rectangular corners of the connecting portions between the ends of the channels hinder the flow of hydrogen or fuel and oxygen by creating turbulence and disrupting a smooth laminar flow.